This invention relates generally to organic photoconducting devices and, in particular, to a photoreceptor having an outer layer comprising minute pockets of silicone oil, particularly in the form of vesicles dispersed in a film forming polymer matrix.
In electrophotography, a photoreceptor containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The photoreceptor is then exposed to a pattern of activating electromagnetic radiation, such as light. The radiation selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided toner particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the photoconductor to a support, such as transparency or paper. This imaging process may be repeated many times.
Photoreceptors may be provided in a number of forms. For example, the photoreceptor may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and at least one other material. One type of composite imaging member comprises a layer of finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. In U.S. Pat. No. 4,265,990, a layered photoreceptor is disclosed having separate photogenerating and charge transport layers. The photogenerating layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer.
Other composite photoreceptors have been developed having numerous layers, which are highly flexible and exhibit predictable electrical characteristics within narrow operating limits to provide excellent images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, a charge transport layer and a conductive ground strip layer adjacent to one edge of the imaging layers. This photoreceptor may also comprise additional optional layers such as an anti-curl back coating and an overcoating layer.
Photoreceptors are generally exposed to repetitive electrostatographic cycling, which subjects exposed layers thereof to abrasion and leads to a gradual deterioration of the mechanical and electrical characteristics of the exposed layers. For example, repetitive cycling has adverse effects on the exposed surface of the outer imaging layer of the imaging member which is generally the charge transport layer, charge generating layer, overcoating layer, electrographic imaging layer and the like. When blade cleaning is utilized to remove residual toner particles from the imaging surface of photoreceptors, particles often adhere to the imaging surface and form comet shaped deposits during cycling. These deposits cannot be readily removed by blade cleaning and appear as undesirable defects in the final print output.
It has also been discovered that glue particles from wrappers utilized for packaging copy paper often accumulate on the photoreceptor surface and cannot be readily removed by cleaning blades. These deposits form black spots on the final print output. In addition, paper fibers cling to the imaging surface and cause print-out defects that also appear as black spots.
Also, the high contact friction that occurs between the cleaning blade and the imaging surface of the photoreceptor tends to wear both the blade and the imaging surface. Reduction in charge transport layer thickness due to wear increases the electrical field across the layer thereby increasing the dark decay and shortening the electrophotographic service life of the imaging member.
Attempts have been made to compensate for wear of the imaging surface. One method has been to increase the thickness of charge transport layers; however, this causes a decrease in the electrical field, which then alters the photoelectric performance and degrades the copy printout quality, which, in turn, requires more sophisticated equipment to compensate for the thicker charge transporting layer. Moreover, in the above method, the change in transport layer thickness as it wears away alters the electrical properties of the photoreceptor and consequently alters the quality of images formed. Attempts have been made to overcome these problems. However, the solution of one problem often leads to additional problems. The following references disclose various problems associated with attempts to improve the wear of photoreceptors.
U.S. Pat. No. 4,078,927 to Amidon et al., issued Mar. 14, 1978, discloses a planographic printing master comprising an ink releasing photoconductive insulating layer and an ink receptive particulate image pattern. The master may be formed from (1) a block copolymer containing polymeric segments from a siloxane monomer and polymeric segments from a non-siloxane monomer and (2) activator compounds, where appropriate (e.g., see line 65, column 3 through line 41, column 4).
U.S. Pat. No. 4,469,764 to Nakazawa et al., issued Sep. 4, 1984, discloses a photosensitive material for electrophotography comprising a dispersion of a charge generating pigment in a charge transporting medium composed mainly of a polyvinyl carbazole resin, wherein a specific perylene pigment is dispersed and incorporated as a charge generating pigment and a specific benzoquinone. A leveling agent such as polydimethylsiloxane may be added to improve surface smoothness of the photosensitive layer (e.g., see column 4, lines 66 through 68).
U.S. Pat. No. 4,332,715 to Ona et al., issued Jun. 1, 1982, discloses a vinyl resin composition obtained by mixing with the vinyl resin a minor portion of an organopolysiloxane that bears one or more acyloxyhydrocarbyl radicals bonded to silicon in the organopolysiloxane.
U.S. Pat. No. 4,784,928 to Kan et al., issued Nov. 15, 1988, discloses an electrophotographic imaging element in which image transfer properties are improved by heterogeneously dispersing, as a separate phase within the photoconductive surface layer of the element, finely divided particles of an abhesive substance. The abhesive substance is non-conductive and spreadable onto which toner particles adhere less strongly than to the composition of the surface layer in the adhesive substance. Various specific materials are disclosed in column 3, lines 1 through 34, including poly(dimethylsiloxane) liquids.
U.S. Pat. No. 4,340,658 to Inoue et al., issued Jul. 20, 1982, and U.S. Pat. No. 4,388,392 to Kato et al., issued Jun. 14, 1983, disclose a photosensitive layer in which surface smoothness may be improved by the addition of a leveling agent such as polydimethylsiloxane to a polyvinyl carbazole type photoconductor.
U.S. Pat. No. 4,738,950 to Banier et al., issued Apr. 19, 1988, discloses a dye-donor element for thermal dye transfer comprising a support having on one side thereof a dye layer and on the other side a slipping layer comprising a lubricating material disposed in a polymeric binder, the lubricating material comprising a linear or branched aminoalkyl-terminated poly-dialkyl, diaryl or alkylaryl siloxane.
U.S. Pat. No. 4,254,208 to Tatsuta et al., issued Mar. 3, 1981, discloses a process for producing a photographic material comprising dispersing in a solution of organic resin, a material that is incompatible with the organic resin to form a dispersion, coating the resulting dispersion on at least one side of a support to form a coated layer, and then drying the coated layer, the material dispersed being a solid at ordinary temperature and in a liquid phase during the dispersing, whereby the coated layer when dried contains solid particular dispersed therein due to solidification of the dispersed material.
U.S. Pat. No. 4,218,514 to Pacansky et al., issued Aug. 19, 1980, discloses an improved waterless lithographic printing master comprising a crosslinked blocked copolymer containing elastic ink releases siloxane blocks chemically linked to organic imaging accepting thermoplastic blocks.
U.S. Pat. No. 3,885,965 to Hughs et al., issued May 27, 1975, discloses a photothermographic element comprising a support having thereon a photothermographic layer comprising a photosensitive silver salt, a polymeric, hydrophobic binder and a poly(dimethylsiloxane).
U.S. Pat. No. 4,519,698 to Kohyama et al. discloses a method in which a waxy lubricant is employed to constantly lubricate a cleaning blade.
U.S. Pat. No. 5,069,993 to Robinette discloses photoreceptor layers containing polydimethylsiloxane copolymers.
U.S. Pat. No. 5,213,928 to Yu, issued May 25, 1993, discloses an electrostatographic imaging member including a supporting substrate and an outer layer on the imaging side of the imaging member, the outer layer including minute spheres of a high molecular weight polysiloxane homopolymer homogeneously dispersed in a continuous film forming polymer matrix. The high molecular weight polysiloxane homopolymer forms pseudo solid pockets dispersed in the film forming polymer. Yu does not disclose any type of silicone oil as the dispersant.
Attempts at reducing the frictional damage caused by contact between the cleaning blade and the photoreceptor include adding a lubricant such as wax to the toner. However, the fixability of the toner may degrade its electrical function, or further filming may occur, resulting in a degraded image.
It is well known in the art that frictional forces can be reduced by applying a lubricant on the surface of the photosensitive drum. U.S. Pat. No. 4,519,698 to Kohyama et al. discloses a method in which a waxy lubricant is employed to constantly lubricate a cleaning blade. However, the thickness of the lubricant film formed on the photosensitive drum is difficult to maintain, and interference with the electrostatic characteristics of the photosensitive member occurs.
Attempts have also been made to construct a cleaning blade with a material having a low coefficient of friction. However, these attempts are subject to the problem of degradation in other characteristics, especially mechanical strength, due to the presence of additives.
According to U.S. Pat. No. 4,340,658 to Inoue et al. and U.S. Pat. No. 4,388,392 to Kato et al., surface smoothness of a photosensitive layer may be improved by the addition of a leveling agent such as polydimethylsiloxane to a polyvinyl carbazole type photoconductor.
When conventional silicone oil was sprayed onto the imaging surface of a charge transport layer to reduce friction, the charge transport layer cracked when bent, even without cycling.
Photoreceptors containing trace quantities of silicone oil are generally known in the art. Trace quantities of silicone oil have been used to affect improvements in surface properties of the dried film and improve flow out during the coating process. The quantity of silicone oil used in these known processes is too small to affect the bulk properties of the dried film. Any lubricating effect of silicone oil on the surface of the known photoreceptors is short lived because the silicone oil is quickly removed during normal abrasive wear due to toner, cleaning blades, brushes, paper contact and the like.
Thus, it is desirable to increase the durability and extend the life of exposed surfaces of a photoreceptor, as well as to reduce frictional contact between members of an organic photoconducting device while maintaining electrical and mechanical integrity and print quality.
U.S. Pat. No. 4,571,371 to Yashiki discloses an electrophotographic photoreceptor having a substrate, electroconductive layer, and a photosensitive layer. The electroconductive layer includes a silicone leveling agent, which improves the interfaces between the electroconductive layer and the other layers. However, the silicone leveling agent is contained in an inner layer of the photoreceptor, not in the outer imaging layer or overcoating layer for the purposes of reducing the coefficient or friction with a cleaning blade, lowering surface energy of the photoreceptor, or improving toner image transfer.